1. Field of the Invention
The present invention relates to a method for determining the closing time of an electromagnetic fuel injector.
2. Description of the Related Art
An electromagnetic fuel injector of the type described, for example, in patent application EP1619384A2 may include a cylindrical tubular body having a central feeding channel, which performs the fuel conveying function, and ends with an injection nozzle regulated by an injection valve controlled by an electromagnetic actuator. The injection valve is provided with a pin, which is rigidly connected to a mobile keeper of the electromagnetic actuator to be displaced by the action of the electromagnetic actuator between a closed position and an open position of the injection nozzle against the bias of a closing spring. The spring pushes the pin into the closed position. The valve seat is defined by a sealing element, which is disc-shaped, inferiorly and fluid-tightly closes the central duct of the supporting body and is crossed by the injection nozzle. The electromagnetic actuator comprises a coil, which is arranged externally about the tubular body, and a fixed magnetic pole, which is made of ferromagnetic material and is arranged within the tubular body to magnetically attract the mobile keeper.
Normally, the injection valve is closed by effect of the closing spring which pushes the pin into the closed position. In the closed position, the pin presses against a valve seat of the injection valve and the mobile keeper is distanced from the fixed magnetic pole. In order to open the injection valve, i.e. to move the pin from the closed position to the open position, the coil of the electromagnetic actuator is energized to generate a magnetic field that attracts the mobile keeper towards the fixed magnetic pole against the elastic force exerted by the closing spring. The stroke of the mobile keeper stops when the mobile keeper itself strikes the fixed magnetic pole.
As shown in FIG. 3, the injection law (i.e. the law which binds the piloting time T to the quantity of injected fuel Q and is represented by the piloting time T/quantity of injected fuel Q curve) of an electromagnetic injector can be split into three zones: an initial no opening zone A, in which the piloting time T is too small and consequently the energy which is supplied to the coil of the electromagnet is not sufficient to overcome the force of the closing spring and the pin remains still in the closed position of the injection nozzle; a ballistic zone B, in which the pin moves from the closed position of the injection nozzle towards a complete opening position (in which the mobile keeper integral with the pin is arranged abutting against the fixed magnetic pole), but is unable to reach the complete opening position and consequently returns to the closed position before having reached the complete opening position; and a linear zone C, in which the pin moves from the closed position of the injection nozzle to the complete opening position, which is maintained for a given time.
The ballistic zone B is highly non-linear and, above all, has a high dispersion of the injection features from injector to injector. Consequently, the use of an electromagnetic injector in ballistic zone B is highly problematic, because it is impossible to determine the piloting time T needed to inject a quantity of desired fuel Q with sufficient accuracy.
A currently marketed electromagnetic fuel injector cannot normally be used for injecting a quantity of fuel lower than approximately 10% of the maximum quantity of fuel which can be injected in a single injection with sufficient accuracy. Thus, 10% of the maximum quantity of fuel which can be injected in a single injection is the limit between ballistic zone B and linear zone C. However, the manufacturers of controlled ignition internal combustion engines (i.e. engines that work according to the Otto cycle) require electromagnetic fuel injectors capable of injecting considerably lower quantities of fuel, in the order of 1 milligram, with sufficient accuracy. This requirement is due to the observation that the generation of polluting substances during combustion can be reduced by fractioning fuel injection into several distinct injections. Consequently, an electromagnetic fuel injector must also be used in ballistic zone B because only in the ballistic zone B can injected quantities of fuel be in the order of 1 milligram.
The high dispersion of injection features in ballistic zone B from injector to injector is mainly related to the dispersion of the thickness of the gap existing between the mobile keeper and the fixed magnetic pole of the electromagnet. However, in light of the fact that minor variations to the thickness of the gap have a considerable impact on injection features in ballistic zone B, it is very complex and consequently extremely costly to reduce dispersion of injection features in ballistic zone B by reducing the dispersion of gap thickness.
The matter is further complicated by the aging phenomena of a fuel injector which can result in a creep of injection features over time.
Published patent applications WO2010023104A1 and WO2002075139A1 describe a piloting method of an electromagnetic fuel injector which, among other matters, contemplates determining the closing time of the injector by detecting the trend over time of the voltage across a coil of an electromagnetic actuator after the annulment of the electric current circulating through the coil and by consequently identifying a perturbation of the voltage across the coil after the annulment of the electric current circulating through the coil.